Spaceborne laser altimeters typically use modest energy (50 to 100 milliJoules) solid-state laser, large telescopes having apertures of 50 to 100 centimeters in diameter, and high detection thresholds to achieve unambiguous surface returns with few or no false alarms resulting from solar background noise. As a result of this conventional design philosophy, spacecraft prime power and weight constraints typically restrict operations to modest repetition rates on the order of a few tens of Hz which, for a typical earth orbit ground velocity of seven kilometers per second, limits along-track spatial sampling to one sample every few hundred meters. There is great motivation in obtaining higher along-track resolution and/or better cross-track coverage, but achieving this capability through a simple scaling of the laser fire rate or power is not practical from spacecraft. This is especially true of altimeters for use in orbit about other planets where instrument mass and prime power usage is severely restricted. In summary, the conventional high signal-to-noise ratio approach to laser altimetry does not make efficient use of the available laser photons.
First generation altimetric approaches are not well suited to generating the few meter level horizontal resolution and decimeter precision vertical (range) resolution on the global scale desired by many in the Earth and planetary science communities. The first generation spaceborne altimeters are characterized by a laser operating in the infrared (1064 nm) at a few tens of Hz with moderate output energies (50 to 100 mJ), a telescope in the 50 to 100 cm range, and a single element (i.e. non-pixellated) detector that detects and processes multi-photon returns from the surface. On bare terrain, the signal waveforms reflect the slope and surface roughness within the laser footprint (typically several tens of meters in diameter) as well as any false slopes due to pointing error. On Earth, the presence of manmade buildings and volumetric scatterers (such as tree canopies or other vegetation) generally makes waveform interpretation more complex and difficult.
One major challenge to the conventional approach is the sheer number of measurements required over a nominal mission lifetime of two to three years. For example, in order to generate a 5 m×5 m vertical grid map of Mars, which has a mean volumetric radius of 3390 km, over 7 trillion individual range measurements are required, assuming that no ground spatial element is measured twice. In any realistic mission, the actual number of range measurements will be significantly larger since an instrument designed to provide contiguous coverage at the planetary equator would oversample the higher latitudes where the ground tracks are more narrowly spaced. If one were to simply scale conventional approaches, one would clearly face severe prime power, weight, and instrument longevity issues.
A second technical challenge is the high ground speed of the spacecraft (about 3 km/sec for a nominal 300 km altitude Mars orbit) coupled with the need to incorporate a scanner to cover the large area between adjacent ground tracks, especially near the equator. At a nominal altitude of 300 km, for example, the satellite would have an orbital period about Mars of approximately 113 minutes. Thus, a three-year mission would produce 13,910 orbits or 27,820 equator crossings with an average spacing between ground tracks at the equator of 766 meters. The latter spacing corresponds to about 154 resolution elements (˜=5 m) in the cross-track direction between adjacent ground tracks and further implies a minimum cross-track scan angle of about 0.15 degrees. For truly contiguous coverage using a conventional single element detector, these 154 cross-track measurements must be completed in the time it takes the spacecraft to move one resolution element in the along-track direction, or within 1.67 msec. This implies a laser fire rate of 92.4 kHz. Furthermore, a uniformly rotating mechanical scanner, for example, must complete a half cycle of its movement within the same 1.67 msec period, i.e. 300 Hz (18,000 RPM). While alternative non-mechanical scanners, such as electrooptic or acousto-optic devices, are capable of very high scanning speeds and have no moving parts, they fall far short of the angular range requirements, are highly limited in their useful aperture, and require fast high voltage or high RF power drivers.
An additional technical challenge stems from the high laser fire rate and the long pulse time of flight (TOF). At 300 km altitude, the laser pulse completes a roundtrip transit to the surface in 2 msec. Thus, for laser fire rates in excess of 500 Hz, multiple pulses will be in flight simultaneously. In principle, it is easy to associate the correct return pulse with the appropriate outgoing pulse provided the roundtrip satellite-to-surface TOF is known beforehand to well within a single laser fire interval. For the 92.4 kHz rate derived previously, however, approximately 185 pulses would be simultaneously in transit, and it would be necessary to have knowledge of the orbit at the 1.6 km level in order to tie a given surface return to the appropriate output pulse unambiguously. While such a navigation accuracy might be easy to achieve in Earth orbit using either Global Positioning System (GPS) receivers or Satellite Laser Ranging (SLR) to passive reflectors on the spacecraft, it would likely be a much more difficult challenge in orbits about extraterrestrial bodies.
An additional technical problem associated with the longer pulse TOF from orbit is related to “transmitter point-ahead”, i.e. the offset between the center of the laser beam at the surface and where the receiver is looking one 2 msec round trip transit time later. For an unscanned system, the offset due to a 3 km/sec spacecraft ground velocity is only 6 m (slightly more than one resolution element) in the along-track direction and can be easily accommodated, either by a fixed offset of the transmitter in the positive along-track direction or by a modest increase in the receiver field of view (FOV). In the current example, however, the scanner must complete over half a cycle of its scan within the pulse TOF. Thus, the receiver FOV must be opened up to span the full 0.15 degree separation (766 m) between ground tracks in the cross-track dimension while the laser illuminates only a 5 m diameter circle within that FOV and defines the ground resolution element being interrogated. This approach greatly increases the solar background noise incident on the detector during local daytime operations relative to the unscanned case and elevates the laser output energy requirements for good discrimination of the signal. An alternative low noise approach would be to steer the transmitter and receiver independently, which will be discussed in later sections.
The surface return rate of an Earth orbiting altimeter can be increased by two to four orders of magnitude for a given laser output power by emitting the available photons in high frequency (few kilohertz) train of low energy (approximately one milliJoule) pulses as opposed to a low frequency train of high energy pulses and employing single photon detection. This mode of operation reduces the risk of internal optical damage to the laser, thereby improving long-term reliability, and makes the beam inherently more eyesafe to a ground-based observer. In addition, these high return rates can often be accomplished with much smaller telescope apertures. Indeed, if the number of receiver stops per timing channel is limited, the contrast of the terrain signal against the solar-induced noise background is actually enhanced through the use of a smaller receive telescope.